1. Field of the Invention
This invention relates to a new silacyclohexane compound, a method of preparing it, and a liquid crystal composition which contains it, as well as a liquid crystal display element containing said liquid crystal composition.
2. The Prior Art
A liquid crystal display element utilizes the optical anisotropy and dielectric anisotropy of liquid crystal substances. Display methods include the TN mode (twisted nematic mode), the STN, mode (super twisted nematic mode), the SBE mode (super birefringence mode), the DS mode (dynamic scattering mode), the guest-host mode, the DAP mode ("deformation of aligned phase" mode) and the OMI mode (optical mode interference mode). The most common display device has a twisted nematic structure based on the Schadt-Helfrich mode.
The properties required of the liquid crystal substance used in these liquid crystal displays are somewhat different depending on the display method. However, a wide liquid crystal temperature range and stability with regard to moisture, air, light, heat, electric fields, etc., are properties commonly required by all display methods. Furthermore, it is desirable for the liquid crystal material to have a low viscosity, and also to have a short address time, low threshold voltage and high contrast in the cell(s).
Currently, there is no single compound which satisfies all these requirements. In practice, liquid crystal mixtures are used which are obtained by mixing several to more than ten liquid crystal compounds and latent liquid crystal compounds. Because of this, it is also important that the components of a liquid crystal composition mix easily.
Among liquid crystal compounds which can be these components, one of the components conventionally known which has a medium positive dielectric anisotropy and a small anisotropy of the refractive index .DELTA.n is a compound which has the bicyclohexyl ring structure such as represented by the general formula shown below. ##STR3##
Also, the compounds with the bicyclohexyl ring structure represented by the general formulas shown below are known to have near zero dielectric anisotropy while their .DELTA.n is small and their viscosity is low. ##STR4##
In recent years, along with the expansion of the applications of liquid crystal displays, the characteristics required of liquid crystal materials are becoming more and more advanced and demanding. In particular, superior characteristics such as a faster response time and a wider angle of visibility are increasingly in demand.
In order to shorten the response time, it is necessary to lower the viscosity of the liquid crystal composition. To widen the angle of visibility, it is required to have an optimum first minimum of the retardation, i.e. to reduce the anisotropy of the refraction index.